In many fluid systems, such as transmissions having fluid actuated clutches, a problem exists in ensuring that the load piston of the modulating relief valve is fully reset prior to initiating engagement of the respective clutches during a shift from one gear to another gear. This problem may be associated with the fact that the system pressure does not have ample time to decrease before the respective clutch is filled and clutch engagement is again initiated or the problem may be caused by the clutch fill pressure remaining to high due to internal resistance to fluid flow. The internal resistance to fluid flow may be caused by cold oil, to much fluid flow from the source, or by the internal passages in the fluid system being to restrictive. When the load piston does not fully reset, the higher initial engagement pressure in the system creates harsh clutch engagement or other undesirable shift dynamics which are detrimental to the life of the clutch and other associated elements in the transmission. It is advantageous to provide a system that ensures full reset of the load piston prior to the new clutch being filled and the pressure begins to increase. Furthermore, in order to control overall costs, the system should be simple in construction and adaptable to other existing systems.
Various arrangements have been used in the past in an effort to minimize the above-noted problems. One such arrangement is disclosed in U.S. Pat. No. 4,751,866 which issued June 21, 1988 to W. W. Blake and assigned to the same assignee as the subject invention. This patent teaches a control system for a transmission having directional and speed clutches in which the speed clutches are engaged first and subsequently followed by engagement of the directional clutches. In this arrangement, during a speed change, the fluid to one of the clutches must be vented and another clutch subsequently filled and pressurized. While the venting and filling of the respective clutches are occurring, the load piston of the modulating relief valve must move to its fully unloaded condition in order to provide smooth engagement of the clutches upon repressurization thereof. Since resetting of the load piston must happen very quickly, numerous problems may exist during a shift. The ratio valve of the above-noted patent is designed so that when the pressure of the directional clutches is three times that of the pressure acting on the load piston, the ratio valve will move to a position in which the load piston is vented to the reservoir. This enables the load piston to quickly reset. However, it has been found at times that the ratio valve does not move to the full open position but merely moves to a throttling position, thus not allowing enough time to fully vent the load piston pressure chamber. Consequently, the pressure in the system does not reach its minimum level and results in harsh engagement of the directional clutches during initial reengagement thereof.
U.S. Pat. No. 3,181,394 which issued on May 4, 1965 to C. A. Ramsel, et al. teaches a control system for a transmission having directional and speed clutches. This system includes a modulating pressure relief valve having a load piston which controls the rate of pressure rise in the clutch actuator during engagement of the respective clutch. In this arrangement, a quick response valve is illustrated which moves to a quick vent position once the pressure there-across reduces to a predetermined value. This arrangement also requires that the system pressure reduces to some predetermined level prior to opening of a vent to unload the load piston pressure chamber. Consequently, there are times during the operation of this system that the load piston may not fully be unloaded prior to engagement of the desired clutch.
U.S. Pat. No. 3,389,770 issued June 25, 1968 to K. F Golan, et al. and assigned to the same assignee as the subject invention teaches a control system having directional and speed clutches. In this arrangement, the system also includes a modulating pressure relief valve having a load piston for controlling the rate of pressure rise in the system during engagement of the respective clutches. Based upon obtaining a predetermined pressure differential in the system, a quick response valve is provided to open during filling of the clutches. The opening of the quick response valve allows the load piston to be reset more quickly. However, there may be times during a speed change that the desired pressure differential in the system is not achieved quick enough to allow the quick response valve to function.
U.S. Pat. No. 3,991,865 issued Nov. 16, 1976 to M. Komatsu teaches a control system for a transmission having pressure applied clutches and a modulating pressure relief valve to control the rate of pressure rise in the system during engagement of the respective clutches. This arrangement has a quick return valve that operates in response to a predetermined differential pressure being obtained in the system during a change in operation from one clutch to another to allow the new clutch to quickly fill prior to starting of a gradual pressure increase.
U.S. Pat. No. 4,132,302 issued Jan. 2, 1979 to P. K. Chatterjea teaches a control system for a transmission having directional and speed clutches. This system includes a load piston which controls the rate of pressure rise in the respective clutch actuators. This arrangement also has a reset piston which automatically connects the load piston pressure chamber with the drain for quick load piston resetting. The reset piston is responsive to the pressure in the clutch actuators. The pressure in the clutch actuators must be reduced to the level equivalent to the clutch fill pressure in order for the reset piston to quickly respond and unload the load piston. Consequently, if the clutch is filled before the system pressure reaches the required reduced pressure or if the fill pressure is to high, the load piston does not reach the full reset position. This results in harsh clutch engagement.
U.S. Pat. No. 4,676,348 issued June 30, 1987 to A. R. Coutant and assigned to the same assignee as the subject invention teaches a control system for a transmission having directional and speed clutches. This arrangement teaches a system wherein an accumulator valve is provided to establish a time delay in the system. Once the system pressure on the clutches reaches a predetermined level, the accumulator piston holds the pressure for a predetermined amount of time and then allows the system pressure operating on the clutches to be reduced in order to save horsepower.
The present invention is directed to overcoming one or more of the problems as set forth above.